Most companies and public institutions construct their own computer networks and perform document transmission or approval between terminals.
As development of communication technologies, working from home or working from outside the office, namely, performing company's work from outside the company, increases. Outside workers access the company's computer network in real time or intermittently from outside the company, download/upload specific documents, or approve specific documents.
Such working from outside the office causes security of the company's computer network to be vulnerable. That is, even though a security system is constructed, malicious accesses by corporate spies or hackers from outside may increase in a state where the company's computer network is open.
In particular, special security management is necessary for the military that treats national security or a nuclear power plant where even a minor malfunction is not allowable. When a hacker intrudes a military computer network, and launches a missile or implants a malicious program to allow important information of the state to be continuously leaked, this is fatal to national defense. On the other hand, situations where savings of customers are unfairly withdrawn occur in the financial companies. This is also caused by lax security management for the computer network.
In order to address these issues, a security management system called as firewall is commonly employed. The firewall system is a hardware and/or software security management system installed to prevent unauthenticated accesses to a computer network. When an external terminal attempts to access a center server, the firewall system provides a communication path allowing only an external device having a pre-registered IP address to access the center server. Some of firewall technologies use packet filtering and the packer filtering determines whether to pass or discard packets on the basis of source and destination IP addresses of the incoming and outgoing packets.
As another solution, there has been endeavored to block malicious access from outside by separating an external network from an internal network and interposing a relay server therebetween.
However, the malicious access is difficult to track down. Even though the malicious access is detected, once the malicious access is performed, a malicious program remains in the relay server and even in the center server. Accordingly, the center server as well as the relay server is required to be replaced or reformatted. This causes loss or damage in important information stored in the center server and results in not only significant expense loss but also a large obstacle to work.